1. Field of the Invention
The present invention relates to a copying machine, a printer, and other image forming apparatuses using an electrophotographic system and an electrostatic recording system, particularly to an image forming apparatus using a magnetic brush charging system.
2. Related Background Art
FIG. 15 is a schematic view showing one example of a transferring system electrophotographic apparatus (a copying machine, a printer, a facsimile machine, or the like) as a conventional example of an image forming apparatus.
A rotary drum type electro photosensitive body (hereinafter referred to as the photosensitive drum) 111 is a first image bearing member and is rotated/driven in a clockwise direction shown by an arrow with a predetermined peripheral speed.
The photosensitive drum 111 is subjected to a uniform charging process with a predetermined uniform polarity and potential by charging means 112 in its process of rotation. The charging means 112 is a charging roller as a contact charging member in this example. Subsequently, the photosensitive drum receives image exposing light L by image exposing means (original image projection exposing means, laser scan exposing means, and the like)(not shown) is received. In this case, electricity is removed (or potential is attenuated) selectively from the uniformly charged surface of the photosensitive drum 111 in accordance with an exposed image pattern, and an electrostatic latent image is formed on the surface of the photosensitive drum 111.
Subsequently, the electrostatic latent image is developed as a toner image by developing means 113.
On the other hand, a transferring material (transferring sheet) P as a second image bearing member is fed to a transferring portion between the photosensitive drum 111 and the transferring means 114 from a sheet feeding mechanism (not shown) at a predetermined control timing, and the toner image on the surface of the photosensitive drum 111 is successively transferred to the surface of the fed transferring material P. The transferring means 114 is a transferring roller in this example.
Subsequently, the transferring material P is detached from the surface of the rotary photosensitive drum 111, introduced to fixing means (not shown), subjected to a fixing process of the toner image and outputted as an image formed material (copy, print).
After the toner image is transferred to the transferring material P, the surface of the photosensitive drum 111 is cleaned by a cleaning apparatus (cleaner) 115, a transfer residual toner is removed from the surface, and the photosensitive drum is repeatedly used for image formation.
1) Contact Charging Device
In the aforementioned image forming apparatus, as the photosensitive drum 111, and the respective means/apparatuses 112 to 115 for the image forming processes such as charging, exposing, developing, transferring, cleaning, and fixing, there are various systems/constitutions.
For example, as the charging means 112 for uniformly charging the surface of the photosensitive drum 111 with the predetermined polarity/potential, a corona charging device has been generally used. This corona charging device is disposed opposite to the photosensitive drum in a non-contact manner, and the surface of the photosensitive drum is exposed to a corona shower generated from the corona charging device with a high pressure applied thereto so that the surface of the rotary photosensitive drum is charged to provide the predetermined polarity/potential.
In recent years, a contact charging device has been put into practical use because of its advantages such as low ozone and low power rather than the corona charging device.
For the contact charging device, an electrically conductive member with an adjusted resistivity is used as a contact charging member and is brought in contact with a body to be charged, a predetermined voltage (charging bias) is applied to the contact charging member, and the surface of the body to be charged is charged to provide the predetermined polarity/potential.
As the contact charging member, a roller type constituted of a rolled electrically conductive rubber (charging roller, electrically conductive rubber roller), a blade type constituted of an electrically conductive rubber blade (charging blade), a magnetic brush type using magnetic particles, a fur brush type constituted by forming electrically conductive fiber in a brush shape, and other various types are preferably used.
For the magnetic brush charging device, electrically conductive magnetic particles are directly carried by a magnet or magnetically bound/held on a sleeve incorporating the magnet as a magnetic brush, the magnetic brush portion of the magnetic particles is stopped or rotated to contact the surface of the body to be charged, and a voltage is applied to the surface to contact-charge the surface of the body to be charged, which is preferably used from the viewpoint of the stability of charging/contact.
For the charging bias to be applied to the contact charging member, there are a DC bias application system in which only a direct-current voltage is used, and an AC bias application system in which a vibration voltage containing a direct-current bias component and an alternating bias component is used.
2) Injection Charging
The contact charging includes a system in which the charging by a discharge phenomenon is dominant as disclosed in Japanese Patent Publication No. 3-52058 or the like, and a system in which the charging by direct injection (charging) of an electric charge with respect to the surface of the body to be charged is dominant as disclosed in Japanese Patent Application Laid-open No. 6-3921 or the like (charge injection charging system).
In the electric charge injection charging system, the contact charging member described above is used, a charge injection charging type is used as the body to be charged, and a usual organic photosensitive body provided with a surface layer with electrically conductive fine particles dispersed therein, an amorphous silicon photosensitive body, or the like is used as the image bearing member, so that the charging potential substantially equal to that of the direct-current component of the bias applied to the contact charging member can be obtained on the surface of the body to be charged.
In the charge injection charging system, since the discharge phenomenon effected by using the corona charging device is not utilized during the charging of the body to be charged, the charging bias required to be applied is only for the desired surface potential of the body to be charged. A completely ozone-less charging in which no ozone is generated, and a low power consumption charging are possible, and the system has been noted.
In the image forming apparatus of the magnetic brush contact charging system and the transferring system, when image formation is repeated, contamination of the magnetic particles of the magnetic brush as the contact charging member occurs, and deterioration of charging property is seen.
The contamination of the magnetic particles of the magnetic brush is caused when toner particle resin components are fused to adhere to the magnetic particles because of a usually relatively high electric resistance of the toner particles, or when an external application agent applied to the toner particles from the outside adheres to the toner particles. This phenomenon raises the resistance of the magnetic particles, the image bearing member as the body to be charged cannot be charged to provide a desired potential, unevenness of charging occurs, and an image defect is generated.